A Handover (HO) process proposed in Institute of Electrical and Electronics Engineers (IEEE) 802.16e mobile Worldwide Interoperability for Microwave Access (WiMAX) includes a scanning process for measuring signals of a serving Base Station (BS) and a neighbor BS, a pre-HO process for determining a target BS of a Mobile Station (MS) to perform HO, and a HO execution process for releasing a connection with the serving BS and resetting a connection with the target BS. By periods or if a signal quality of the serving BS is reduced to a predetermined level or less, the MS sends a scanning request message to the serving BS and acquires signal quality information of neighbor BSs through the scanning process. Also, the MS can perform an association process to acquire a physical layer parameter of the neighbor BS and network information during the scanning process.
A conventional HO scheme is described in detail. If a signal quality of a serving BS is reduced to a predetermined level or less, an MS transmits a HO request (MOB_MSHO-REQ) message including signal quality information of neighbor BSs, to the serving BS. Then, the serving BS receives the HO request (MOB_MSHO-REQ) message transmitted by the MS. Alternately, the serving BS compares signal quality information of the neighbor BSs specified in a scanning report (MOB_SCN-REP) message transmitted by the MS with a HO initialization condition and, if signal qualities of the neighbor BSs meet the HO initialization condition, the serving BS transmits HO pre-notification (HO-pre-notification) messages to the neighbor BSs in order to recognize HO acknowledgment or non-acknowledgement for the MS to perform HO. The HO pre-notification (HO-pre-notification) message includes information on a wireless resource (i.e., bandwidth) and service class (i.e., Quality of Service (QoS) level) required by the MS as well as an estimated HO time of the MS.
Then, the neighbor BSs transmit HO pre-notification response (HO-pre-notification-response) messages including information on HO acknowledgement or non-acknowledgement and a serviceable wireless resource and service class, to the serving BS. Then, the serving BS transmits a HO response (MOB_BSHO-RSP) message to the MS and transmits a HO pre-notification confirm (HO-pre-notification-confirm) message to the neighbor BS that sends an acknowledgement response for a HO of the MS. The HO response (MOB_BSHO-RSP) message includes information on neighbor BSs sending acknowledgement responses for HO of an MS, and information on a wireless resource and service class serviceable by each of the neighbor BSs, an estimated HO action time, and a resource retain time for retaining a wireless resource allocated by a serving BS to the MS.
Then, the MS transmits a HO indication (MOB_HO-IND) message of informing performance of final HO to a target BS, to the serving BS. After the resource retain time, the MS releases a connection setup with the serving BS. Then, the MS receives a signal for downlink channel synchronization and a DownLink/UpLink MAP (DL/UL-MAP) from the target BS to reset a connection with the target BS, performs a ranging process with the target BS to be allocated a Connection IDentifier (CID) and, during a corresponding period, sets a magnitude of a UL transmit signal, thus resetting a connection with the target BS.
In order to reduce a HO delay caused by ranging of a HO MS, the target BS can transmit a fast ranging Information Element (IE) to the MS at the estimated HO time. However, the estimated HO time is forwarded from the serving BS only to the neighbor BS. Thus, it may occur that the MS cannot consider the conditions of the target BS that is a target for HO and in addition, the MS fails to receive the fast ranging IE transmitted by the target BS if a pre-HO delay that is a time for which the MS prepares HO increases because of an increase of a delay caused by a loss of the HO response (MOB_BSHO-RSP) message resulting from attenuation of a signal strength of the HO MS, a backbone network delay between the serving BS and the target BS, and a queuing delay caused by a traffic increase of the serving BS. This may result in an increase of a time for which the MS cannot be served during HO performance, that is, a HO interruption time for which normal communication performance after a release of a connection with the serving BS is impossible, thus causing the deterioration of a QoS for the MS.
As a solution to this, the estimated HO time increases. In such event, a delay necessary for receiving the fast ranging IE also increases. Therefore, in like manner, a problem exists in that the HO interruption time may not only increase but also a physical layer parameter of the target BS acquired in an association process may be changed, thus causing a failure of fast ranging. Additionally, a problem exists in that, if the serving BS fails to successfully transmit traffic transmitted to the MS before a release of a connection setup while performing HO for the MS after transmitting the HO pre-notification messages to the neighbor BSs, a packet loss may occur, or a decrease of a QoS may occur because of a delay caused by tunneling from the serving BS to the target BS.